This invention relates to light sources. More particularly, it relates to a light source having a relatively high light output and a low temporal coherence.
Light emitting diodes (LED's) and semiconductor laser diodes are representative of light sources currently used for introducing light energy into an optical fiber. In light emitting diodes, photons of a characteristic wavelength are emitted when electrons and holes recombine across a forwardly biased junction defined by semiconductors of opposite conductivity type. The wavelength of the emitted photons is distributed in a generally Gaussian manner about a central wavelength determined by the nature of the semiconductor materials, their dopants and the temperature. Since the electron/hole recombinations occur randomly, the light output is essentially incoherent, that is, the light output has very low temporal coherence. Light emitting diodes are suitable for use as light sources in some optical fiber systems, since their small size allows convenient coupling to an optical fiber core and they can be switched fast enough to be useful for many digital data transmission applications. However, the radiant intensity of the light provided by most LED's, even the so-called super-radiant LED's, is generally insufficient for many optical fiber applications, including some optical fiber gyroscopes. In addition, the wavelength of the light produced by LED's is temperature dependent, this aspect limiting their use to applications where wavelength stability is not a primary consideration.
Semiconductor laser diodes, in contrast to LED's, include structure that defines a resonant cavity in which light emitted along a principal axis can oscillate within the boundaries of the cavity to create a standing wave pattern that causes stimulated light energy emission. The light output exiting one end of the cavity has a high radiant intensity and temporal coherence as well as a narrow spectral line width compared to the light output of an LED. Like the LED, the wavelength of the light output of a laser diode can be temperature dependent.
The laser diode is well suited for optical fiber communication systems because of its high light output. However, the relatively high temporal coherence of the laser diode light output, as well as the narrow spectral line width, can cause undesirable scattering effects, including interference effects from Rayleigh back scatter. One optical fiber device in which interference with scattered light is not desirable is the optical gyroscope. In an optical gyroscope, a two-port optical fiber circuit is provided in which light energy from a common source is inputted into each port to provide counter-travelling light paths. The light energy output at each end of the optical fiber is then compared to determine the phase difference between the counter-travelling paths. Where the system is subjected to a relative spatial rotation at some rate, the effective path length of one of the light paths appears longer and the effective path length of the other of the light paths appears shorter to provide an apparent relative path length difference. The apparent path length difference can be detected by observing the phase difference of the light propagation that are representative of the angular rate. Since the phase differences are small, any interference effects due to scattered light can obscure the rate-dependent phase difference information.
Where an LED is utilized as the light source for an optical fiber gyroscope, the radiant intensity of the light energy, after it is coupled into the fiber circuit, is low enough such that a high noise-to-signal ratio exists which can make difficult the detection of very low angular rates, as the information bearing signal can be obscured by noise. On the other hand, where a laser diode is utilized as the light source, the relatively high temporal coherence combined with the characteristically narrow spectral line width can produce various types of interference between the primary beams and the scattered light that can likewise make interpretation of the rate signal difficult. With a super radiant LED, the light intensity coupled into a single mode fiber is higher than with an ordinary LED, but is not as high as with a laser diode and, furthermore, has temporal coherence which tends to be intermediate in value between an LED and laser diode. Additionally, the temperature dependence of the wavelength of the output of an LED, super radiant LED or laser diode does not contribute to optimum device operation.
As can be appreciated, a need exists for a light source that is small in size, has a high radiant intensity output, a relatively low temporal coherence, and smoothly distributed line-free and temperature independent spectral characteristics to provide a light source well suited for many applications, such as in equal path interferometers, of which the fiber optic gyroscope is an example.